The present section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present disclosure that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Standard-Dynamic-Range frames (SDR frames) are frames whose luminance values are represented with a limited number of bits (most often 8 or 10). This limited representation does not allow correct rendering of small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range frames (HDR frames), the signal representation is extended in order to maintain a high accuracy of the signal over its entire range. In HDR frames, pixel values are usually represented in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or in integers with a long representation, typically at least 16 bits.
A typical approach for encoding a HDR frame is to reduce the dynamic range of the frame in order to encode the frame by means of a legacy encoding scheme (initially configured to encode SDR frames).
According to a well-known approach, a backlight frame is determined from the luminance component of the input HDR frame. A residual frame is then obtained by dividing the input HDR frame by the backlight frame and both the backlight frame and the residual frame are encoded by a legacy encoder such as H.264/AVC ((“Advanced video coding for generic audiovisual Services”, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Recommendation ITU-T H.264, Telecommunication Standardization Sector of ITU, January 2012).) or HEVC, SHVC (“High Efficiency Video Coding”, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Recommendation ITU-T H.265, Telecommunication Standardization Sector of ITU, October 2014).
Such an encoding approach automatically generates a SDR frame by dividing the HDR frame by a backlight frame obtained from the luminance component of the HDR frame.
Consequently, such an encoding approach is not adapted when a SDR frame is imposed, i.e. is an input of the encoding and shall also be encoded, because there is virtually no chance to get an automatic low-dynamic range frame from the HDR frame close enough (in term of visual content) to the imposed SDR frame.
This occurs, for example, in cinema-oriented use cases where it is expected to obtain from a studio two different color grading: one for a SDR frame obtained from the HDR frame and one for the HDR frame itself. It is, then, virtually impossible from the usual backlight-based method for encoding a HDR frame to get an automatic SDR frame (by dividing the SDR frame by a backlight frame) close enough (in term of visual content) to the artistically graded SDR frame.